In older oil-producing fields, water channeling through the high permeability zones in an oil reservoir will by-pass a large amount of oil-in-place. The more permeable zones of the subterranean formation tend to take most of the injected fluids. While initially this is acceptable in sweeping oil from such zones of relatively high permeability, it subsequently becomes undesirable as the oil content of such strata becomes depleted since much of the subsequently injected floodwater or other fluid by-passes the relatively less permeable zones and provides little benefit in enhancing further hydrocarbon recoveries.
Indepth plugging of a relatively high-permeability zone is to be preferred, so as to convert the zone into a much lower permeability zone. Then, subsequently injected floodwater will tend to enter the previously by-passed but now relatively more permeable hydrocarbon bearing zones and thus mobilize increased amounts of hydrocarbons therefrom.
A variety of means of subterranean formation permeability corrections have been developed. U.S. Pat. No. 3,762,476 (Gall, Oct. 2, 1973) describes the use of a variety of polymers in conjunction with crosslinking solutions of various multivalent metal cations complexed with certain sequestering/retarding anions selected from the recited group consisting of acetate, nitrilotriacetate, tartrate, citrate, and phosphate. The crosslinking solution is injected after the first polymer-thickened solution injection, frequently after and before interspacing brine slugs, followed by injection of further polymer solution, which sequence can be repeated. U.S. Pat. No. 4,018,186 (Gall et al, Apr. 19, 1977), describes similar materials further with employment of controlled pH.
However, many of the materials have been deficient in hard brines (brines containing calcium and/or magnesium ions) in regard to providing a high level of effectiveness on a large scale, avoiding undue sedimentation and strata plugging, inadequate formation of gel, and the like.
A problem with bacteria also has become increasingly apparent in oil field work. Bacteria in the available feedstock surface water and in the deep subterranean structures, particularly sulfate reducing bacteria, do exist and thrive under what had seemed very adverse conditions. Sulfate reducing and other bacteria encountered in subterranean strata have become increasingly troublesome because of the production of hydrogen sulfide which causes corrosion, "sours" the oil in place and causes the precipitation of insoluble compounds such as iron sulfide which can cause the undesirable plugging of pores in the formation.
Needed have been treatments with materials which are effective as crosslinking agents and also possess bactericidal properties.
Heretofore, of the sequestered cationic materials employed in gelling polymers, aluminum citrate solutions have been the crosslinking agents of preference for economy and availability. However, a real problem has persisted in that to make up the aluminum citrate solutions fresh water must be used. Since tremendous volumes of injection solutions are involved in flood work, providing large quantities of fresh water of injection quality at the well sites has indeed been a serious problem in many locations.
Needed are effective crosslinking agents, which can be used in hard brine solutions, and at the same time possess biocidal properties particularly toward sulfate reducing bacteria.